How to remove unreferenced UV Coordinates from a Texture
The geometry is reflected on the UV. If you mention it is junk, probably delete the mesh junk park associated with it as well. Otherwise, re-unwrap Uvs.

I read this question 3 times. I know what you´re having: shadow uvs. Yeah, bisecting is a special boolean case. I´d strongly recomend you re-unwrap. It will only take a second. Baking textures: yes, you can do that as well, you´ll also get the old UV set. Got to object properties> vertex groups> Uv coordinates, delete them both vertex groups and coordinates. Then Un-wrap. Then bake old bisected to new unwrapped. Should work out of the box.